1. FIELD OF THE INVENTION
This invention relates to photothermographic silver halide materials as well as a method of preparing such materials. In one of its aspects it relates to a photothermographic silver halide composition capable of being coated comprising photosensitive silver halide and other components with a specific combination of solvents.
2. DESCRIPTION OF THE STATE OF THE ART
Photothermographic materials are well known in the photographic art. Photothermographic materials are also known as heat developable photographic materials. The photothermographic materials after imagewise exposure are heated to moderately elevated temperatures to produce a developed image in the absence of separate processing solutions or baths. The heat development can provide a developed silver image in the photothermographic material.
The term "material" as used herein, such as in photothermographic material, is intended to include elements and compositions. For instance, the use of "photothermographic material" is intended to refer to photothermographic element and photothermographic composition.
An example of a known photothermographic silver halide material comprises (a) a hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer with (b) an organic solvent mixture, (c) a hydrophobic binder and (d) an oxidation-reduction image-forming composition comprising (i) a silver salt of a long-chain fatty acid, such as silver behenate or silver stearate, with (ii) an organic reducing agent, such as a phenolic reducing agent. It has been desirable to have hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer in such a photothermographic material because of the higher photosensitivity of the silver halide emulsion and the ease of control in preparation of the emulsion based on conventional aqueous silver halide gelatino emulsion technology. A problem has been encountered in preparing such photothermographic silver halide materials. This problem involves the mixing of a hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer with a composition, as described, containing hydrophobic components including a hydrophobic binder, such as poly(vinyl butyral), and a silver salt of a long-chain fatty acid, such as a silver salt of behenic acid. Typically, when a hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer is mixed with such hydrophobic materials and then coated on a suitable support to produce a photothermographic element, the resulting photothermographic element produces a less than desired degree of photosensitivity, contrast and maximum density upon exposure and heat processing.
This problem has been encountered in photothermographic silver halide materials, as described in, for example, U.S. Pat. No. 3,666,477 of Goffe, issued May 30, 1972. Goffe proposed addition of alkylene oxide polymers and a mercaptotetrazole derivative to the photothermographic material to help provide increased photosensitivity.
In addition, a variety of organic solvents have been proposed in order to help prepare a photothermographic silver halide composition containing the described components. Such organic solvents that have been proposed include isopropanol, acetone, toluene, methanol, 2-methoxyethanol, chlorinated solvents, acetone-toluene mixtures and certain non-aqueous polar organic solvents. These solvents in photothermographic materials are described in, for example, U.K. Specifications Nos. 1,422,145; 1,460,868; and 1,354,186. The described individual solvents, such as isopropanol, have not provided the desired improved described properties. There has been a continuing need to provide improved relative speed and contrast with desired maximum image density.